UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Aerial blocking circuit for TRFs anybody any ideas

Skywave · 17th Feb 2020, 2:18 pm

What you need is what is known as a 'rejector circuit' in the aerial down-lead, fitted close to the radio. That cct. is nothing more than a coil in parallel with a capacitor - ideally a variable one, initially, until the precise value can be found by experiment and consequent measurement and a fixed vale of capacitor fitted instead. Having said that, if the variable is left in place, no harm will be done.

The resonant freq. of the L/C cct. should equal the freq. you wish to reject. Such a parallel tuned cct. presents a high impedance to that freq. It will be desirable to aim at a fairly high Q, so the L/C ratio needs to be at least 100. Use an air-spaced coil with a large diameter to length ratio. A coil diameter of about 3" would be a good start.
If you have access to a Q-meter, then the whole job will be a lot easier.

Al.

17th Feb 2020, 2:18 pm	#6
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Aerial blocking circuit for TRFs anybody any ideas What you need is what is known as a 'rejector circuit' in the aerial down-lead, fitted close to the radio. That cct. is nothing more than a coil in parallel with a capacitor - ideally a variable one, initially, until the precise value can be found by experiment and consequent measurement and a fixed vale of capacitor fitted instead. Having said that, if the variable is left in place, no harm will be done. The resonant freq. of the L/C cct. should equal the freq. you wish to reject. Such a parallel tuned cct. presents a high impedance to that freq. It will be desirable to aim at a fairly high Q, so the L/C ratio needs to be at least 100. Use an air-spaced coil with a large diameter to length ratio. A coil diameter of about 3" would be a good start. If you have access to a Q-meter, then the whole job will be a lot easier. Al.